The invention relates to a sensor assembly for a motor vehicle. In particular, the invention relates to a sensor device having a control and evaluation device, and at least one sensor electrode coupled to this control and evaluation device. The sensor electrode is preferably designed as a conductor loop or as a coaxial cable, which is coupled to the control and evaluation circuit. The control and evaluation circuit has a measuring circuit, which monitors the sensor electrode for capacitance changes.
Sensor assemblies of the type specified above are used in the field of vehicle engineering in order to detect an approach of objects and to design a more comfortable operation of the vehicle.
In particular, sensor devices of this type, with capacitive electrode loops, are used in the rear region of a vehicle. If the sensor electrode extends over a region of the vehicle rear, then a user can trigger an opening of the hatch through merely approaching the sensor device, or executing a gesture in the detection region of the sensor device, for example. During this process, when a user desire has been detected, the identity of the user is queried prior to an opening, which user normally carries a wireless radio key for a wireless querying of the identification.
Circuits and solutions for capacitive sensors on vehicles are available in the market, which enable capacitance detection at variously designed sensor electrodes. By way of example, the change to the capacitance of a sensor electrode is detected in that the sensor electrode is repeatedly coupled to an operating voltage at a defined frequency, and the charging and discharging process is evaluated with regard to a capacitance change. Methods of this type are known, for example, from U.S. Pat. No. 5,730,165. Another capacitive sensor is known from EP 1 339 025 B1.
Some of the known sensor assemblies have a more complex construction, e.g. they are constructed with so-called guard electrodes, and if applicable, with separate ground electrodes, in addition to the sensor electrode. This serves to adjust the detection range of the sensor electrode to the desired operating region as precisely as possible. Relevant disclosures are, by way of example, EP 0 518 836 A1, DE 101 31 243 C1, or DE 10 2006 044 778 A1.
One problem with the known sensor assemblies is, both with the initial installment, and the initial start-up, as well as during long-term operation, ensuring that there is a proper coupling of the sensor electrode to the control and evaluation circuit. With looped sensor electrodes that are coupled to the control and evaluation circuit at two pickup points, e.g. at both ends, a continuity test, for example, is carried out, in order to verify that the sensor electrode is coupled to the control and evaluation circuit such that it can make measurements. This approach is not precise and reliable enough in the framework of available measurement precisions and components.
The object of the invention is to provide an improved and universal sensor assembly, which enables a reliable checking of an integrity of the entire assembly.